<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button>按钮1</button>
    <button>按钮2</button>
    <button>按钮3</button>
    <script>
        var btns=document.querySelectorAll("button");
        var helper=function(x){
            return function(){
                alert(x);
            }
        }
        for (let i = 0; i < btns.length; i++) {
            btns[i].onclick=helper(i);
        }
    </script>    
</body>
</html>